DE-NT0005638

Goal
The goal of this project is to improve estimates of methane flux from submarine seeps and associated gas hydrate deposits on continental margins by compiling a remote sensing inventory of active gas and oil vents and completing sea-truth measurement of flux from representative vents in the Gulf of Mexico.

Proposed study sites GC184 (540m), GC852 (1400m), MC118 (880m) and port of departure (Freeport, TX). Inset lower left shows oil slicks floating over MC118 site.

Performers
Texas A&M University - Corpus Christi, Corpus Christi, TX 78412-5844
University of California San Diego (Scripps Institute of Oceanography) – San Diego, CA
University of California – Santa Barbara, Santa Barbara, CA
Texas A&M University, College Station, TX
University of Southern Mississippi, Stennis, MS

Background
Submarine gas hydrate represents a large pool of greenhouse gas that may interact with the atmosphere over geologic time to affect climate cycles. In the near term, the magnitude of methane reaching the atmosphere from gas hydrate on continental margins is poorly known because 1) gas hydrate is exposed to metastable oceanic conditions in shallow, dispersed deposits that are poorly imaged by standard geophysical techniques and 2) the consumption of methane in marine sediments and in the water column is subject to uncertainty.

A small outcrop of oil-stained gas hydrate on the seafloor of the Gulf of Mexico at 550 m depth (width of image is approx. 2 m). Such deposits are subject to dissolution and off-gassing as the hydrate decomposes

The northern GOM is a prolific hydrocarbon province where rapid migration of oil, gases, and brines from deep subsurface petroleum reservoirs occurs through faults generated by salt tectonics (Roberts and Carney 1997). In the Gulf of Mexico, the focused expulsion of hydrocarbons is manifested at the seafloor by gas vents, gas hydrates, oil seeps, chemosynthetic biological communities, and mud volcanoes (De Beukelaer et al. 2003). Hydrocarbon gas is emitted as bubble plumes from focused gas vents within larger hydrocarbon seep sites. The bubble plumes are visible throughout the water column on acoustic profiles and echo-sounder records (De Beukelaer et al. 2003; MacDonald et al. 1994; Roberts and Carney 1997), and the bubbles are commonly coated with a thin layer of oil (Leifer and MacDonald 2003). Upon reaching the sea surface, this oil forms targets that can be detected by satellite remote sensing methods such as synthetic aperture radar (SAR). SAR imagery shows ~350 perennial oil slicks associated with hydrocarbon plumes offshore Louisiana (De Beukelaer et al. 2003; Leifer and MacDonald 2003; MacDonald and Leifer 2002; MacDonald et al. 2003), and ~100 slicks in the southern GOM (MacDonald et al. 2005; MacDonald et al. 2002). These are minimum estimates that exclude the non-oily plumes in the GOM, which may be equally as abundant as the oily ones (De Beukelaer et al. 2003; Leifer and MacDonald 2003). Recent seismic studies and ground-truth observations estimate that there may be 5,000 geologically active seep sites in the northern GOM (Frye 2008). However, the present day release rates from these sites have not been well-constrained and cannot be confirmed from seismic evidence alone.

Preliminary results of SAR data analyzed for persistent hydrocarbon seeps and gas hydrate deposits showing seep sources in the Green Canyon area of the northern Gulf of Mexico. Each symbol represents a separate SAR image. We estimate that there are over 900 individual sources in this small region of the Gulf of Mexico slope.

Potential Impacts
The project will enhance scientific understanding related to the amount of methane that may be contributed to the atmosphere from oil-associated, deep water, hydro-carbon seep sites and will improve upon understanding of the fate of methane as it transitions through ocean bottom sediment and the water column. The project will also improve upon methods of estimating seep occurrence through the use of satellite imagery and will inform studies of consumption of methane in marine sediments under different geologic regimes. All of these advancements will help provide key data on the fate of marine methane which is a necessary component in the understanding of the contribution that gas hydrates may play in the global carbon cycle and what impact hydrates could have on global climate.

Accomplishments
Project was initiated October 1, 2009. There are no technical accomplishments under the project to date.

Current Status
This project was awarded on September 30, 2008 with a start date of October 1, 2008. The project is to be carried out as a 2 year / 2 Phase effort. Planned activity within Phase 1 is to include:

• Instrument trials and deployment tests (Sonar development and calibration and instrument field testing)
• Planning and Preparation for Field Expedition
• Conducting Research Field Expedition
• Conducting Gulf of Mexico hydrocarbon seep: regional inventory (Image Analysis, database development)
• Initiation of Other region hydrocarbon seep inventory
• Initiation of post expedition analyses ( Bubble Flux Analysis, Water Sample Analysis, Water Column Flux Analysis, Air-Sea Flux Modeling, Sediment Analyses, Pore Water Analyses and Carbonate Analyses)
• Initiation of modeling of AOM and carbonate precipitation rates

Phase 2 is scheduled to initiate in October 2009 and will include completion of the regional hydrocarbon inventories, AOM and carbonate modeling, and physical analyses initiated under Phase 1. Phase 2 will also focus on evaluation and reporting on data and the implications of results achieved.

Project Start: October 1, 2008
Project End: September 30, 2010

Project Cost Information:
Phase 1 - DOE Contribution: $687,352, Performer Contribution: $210,151
Phase 2 - DOE Contribution: $293,359, Performer Contribution: $116,981
Planned Total Funding (if project continues through all project phases):
DOE Contribution: $980,711, Performer Contribution: $327,132

Contact Information: NETL – Traci Rodosta (Traci.Rodosta@netl.doe.gov or 304-285-1345)
Texas A&M University - Corpus Christi – Ian MacDonald (Ian.MadDonald@tamucc.edu or 361-825-5761)

Additional Information
Kick-off Meeting Presentation [PDF - 4.31MB] - December, 2008

Literature Cited
De Beukelaer, S. M., I. R. MacDonald, N. L. Guinnasso and J. A. Murray, 2003. Distinct side-scan sonar, RADARSAT SAR, and acoustic profiler signatures of gas and oil seeps on the Gulf of Mexico slope. Geo-Marine Letters 23(3-4): 177-186.

Frye, M., 2008. Preliminary evaluation of in-place gas hydrate resources: Gulf of Mexico outer continental slope. U.S. Dept. Interior, Minerals Management Service OCS Report: MMS 2008-004.

Leifer, I. and I. R. MacDonald, 2003. Dynamics of the gas flux from shallow gas hydrate deposits: interaction between oily hydrate bubbles and the oceanic environment. Earth and Planetary Science Letters 21(3-4): 411-421.

MacDonald, I. and I. Leifer, 2002. Constraining rates of carbon flux from natural seeps on the northern Gulf of Mexico Slope. VII International Conference on Gas in Marine Sediments and Natural Marine Hydrocarbon Seepage in the World Oceans with Application to the Caspian Sea, Baku, Azerbaijan, NATO and Shallow Gas Group.

MacDonald, I. R., L. C. Bender, M. Vardaro, B. Bernard and J. R. Brooks, 2005. Thermal and Visual Time-Series at a Seafloor Gas Hydrate Deposit on the Gulf of Mexico Slope. Earth and Planetary Science Letters 233: 45-59.

MacDonald, I. R., N. L. Guinasso, Jr., R. Sassen, J. M. Brooks, L. Lee and K. T. Scott, 1994. Gas hydrate that breaches the sea floor on the continental slope of the Gulf of Mexico. Geology 22: 699-702.

MacDonald, I. R., I. Leifer, R. Sassen, P. Stine, R. Mitchell and N. Guinasso Jr., 2002. Transfer of hydrocarbons from natural seeps to the water column and atmosphere. Geofluids 2: 95-107.

MacDonald, I. R., W. W. Sager and M. B. Peccini, 2003. Association of Gas Hydrate and Chemosynthetic Fauna in Mounded Bathymetry at Mid-Slope Hydrocarbon Seeps: Northern Gulf of Mexico. Marine Geology 198: 133-158.

Roberts, H. H. and R. S. Carney, 1997. Evidence of episodic fluid, gas, and sediment venting on the northern Gulf of Mexico continental slope. Economic Geology and the Bulletin of the Society of Economic Geologists 92(7-8): 863-879.